How do I use the iPad touch screen while the Magic Keyboard is connected

When using an external keyboard, the on-screen keyboard is suppressed.

However, should you choose or need to use the on-screen keyboard while an external keyboard is connected, simply tap the keyboard icon that you should see at the bottom of the screen - noting that the position and presentation of the icon may differ between Apps.

By example, when using Safari, you may see this:

Tapping the keyboard icon may expose a secondary menu:

I'm a little confused by your question. Having the keyboard connected does not turn off the touch screen. You should still be able to touch the screen and use the iPad as normal even though the keyboard is connected. The keyboard just gives you additional ways to use your iPad. It will however, disable the onscreen keyboard that appears on the iPad when there is no keyboard attached.